Weather officials in California warned potential beachgoers to stay far away from the ocean until at least Thursday due to a spate of massive waves they said could cause “certain death.”

The National Weather Service for the San Francisco Bay Area issued a high surf advisory on Tuesday, saying waves between 15 to 25 feet would regularly hit the shore until later this week and warning that some waves could top out at 30 feet in some areas. Previous advisories predicted occasional waves of 50 feet or more, and the agency issued a stark message to anyone attempting to brave the surf.

“STAY WELL BACK FROM THE OCEAN OR RISK CERTAIN DEATH,” the NWS for the Bay Area wrote last week.

Further south, the NWS Los Angeles said waves up to 25 feet could cause powerful rip currents and strong seas could damage piers and coastal structures. The office urged inexperienced swimmers and surfers to stay out of the water and told others to stay far back from the water’s edge in case a large wave comes ashore unexpectedly.

Authorities closed the popular Ventura Pier on Tuesday after waves snapped at least one of the structure’s pilings. It’s unclear when the landmark will reopen, but officials remained hopeful that damage would be minimal.